Liquid-crystalline medium

ABSTRACT

A liquid-crystalline mediums which contain one or more compounds of the formula A  
                 
 
     and at least one compound of the formula B  
                 
 
     wherein compounds of formula A contain at least one ring selected from  
                 
 
     and in which R a , R b , ring A and ring B, y, Z 1 , Z 2 , a, L 1 , L 2 , L 3 , L 4 , L 5 , L 6 , L 7 , L 8  and L 9  are as defined herein, are suitable for use in twisted nematic (TN) and supertwisted nematic (STN) liquid-crystal displays. Such displays have very short response times and good steepnesses and angle dependencies.

[0001] The invention relates to liquid-crystalline compounds containinga pyran ring, to a liquid-crystalline medium, to twisted nematic (TN)and supertwisted nematic (STN) liquid-crystal displays having very shortresponse times and good steepnesses and angle dependencies, and to thenovel nematic liquid-crystal mixtures used therein.

[0002] TN displays are known, for example from M. Schadt and W.Helfrich, Appl. Phys. Lett., 18,127 (1971). STN displays are known, forexample from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M.Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals(8.-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadtand F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., JapaneseJournal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F.Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. vanSprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J.Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M.Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P.Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp.1-8 (1986). Theterm STN here covers any relatively highly twisted display elementhaving a twist angle with a value of between 160° and 360°, such as, forexample, the display elements according to Waters et al. (C. M. Waterset al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. DisplayConference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J.Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M.Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs(EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391(20.6)).

[0003] STN displays are distinguished compared with standard TN displaysby significantly better steepnesses of the electro-opticalcharacteristic line and, associated therewith, better contrast values,and by significantly lower angle dependence of the contrast.

[0004] Of particular interest are TN and STN displays having very shortresponse times, in particular also at relatively low temperatures. Inorder to achieve short response times, the rotational viscosities of theliquid-crystal mixtures have hitherto been optimised using mostlymonotropic additives having relatively high vapour pressure. However,the response times achieved were not adequate for every application.

[0005] In order to achieve a steep electro-optical characteristic linein the displays according to the invention, the liquid-crystal mixturesshould have relatively large values for the ratio between the elasticconstants K₃₃/K₁₁ and relatively small values for Δε/ε_(⊥), where Δε isthe dielectric anisotropy and ε_(⊥) is the dielectric constantperpendicular to the longitudinal molecular axis.

[0006] In addition to optimisation of the contrast and response times,further important requirements are made of mixtures of this type:

[0007] 1. broad d/p window

[0008] 2. high long-term chemical stability

[0009] 3. high electrical resistance

[0010] 4. low frequency and temperature dependence of the thresholdvoltage.

[0011] The parameter combinations achieved are still far from adequate,in particular for high-multiplex STN displays (with a multiplex rate inthe region of about 1/400), but also for medium- and low-multiplex STNdisplays (with multiplex rates in the region of about 1/64 and 1/16respectively), and TN displays. This is partly attributable to the factthat the various requirements are affected in opposite manners bymaterial parameters.

[0012] Thus, there continues to be a great demand for TN and STNdisplays, in particular for medium- and low-multiplex STN displays,having very short response times at the same time as a largeworking-temperature range, high characteristic-line steepness, goodangle dependence of the contrast and low threshold voltage which meetthe above-mentioned requirements.

[0013] The invention has the object of providing liquid-crystallinemedia, in particular for TN and STN displays, which do not have theabove-mentioned disadvantages or only do so to a lesser extent and atthe same time have short response times, in particular at lowtemperatures, and very good steepnesses. The media according to theinvention are furthermore suitable for IPS (in-plane switching)applications.

[0014] It has now been found that this object can be achieved if use ismade of liquid-crystal mixtures which comprise one or more compounds ofthe formula A

[0015] and at least one compound of the formula B

[0016] in which

[0017] R^(a) and R^(b) are each, independently of one another, H or analkyl radical having from 1 to 12 carbon atoms which is unsubstituted,monosubstituted by CN or CF₃ or at least monosubstituted by halogen,where one or more CH₂ groups in these radicals may also each,independently of one another, be replaced by —O—, —S—,

[0018]  —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a waythat O atoms are not linked directly to one another,

[0019]  are each, independently of one another,

[0020]  at least one ring is

[0021] Z¹ and Z² are each, independently of one another, —CH₂CH₂—,—(CH₂)₄—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, —C═C—, —CF═CF—, —CH═CH—,—COO—, —CH₂—, —(CH₂)₃— or a single bond,

[0022] a is 0 or 1,

[0023] L¹ to L⁹ are each, independently of one another, H or F, and

[0024] Y is F, C₁, SF₅, NCS, SCN, CN, OCN, or a monohalogenated orpolyhalogenated alkyl, alkoxy, alkenyl or alkenyloxy radical, eachhaving up to 5 carbon atoms.

[0025] The use of compounds of the formula A containing a pyran ring incombination with compounds of the formula B gives, in particular, TN andSTN mixtures which are distinguished over the prior art by relativelyhigh clearing points, good values for the optical anisotropy (Δn) andvery high values for the dielectric anisotropy and by significantlyimproved rotational viscosity.

[0026] The use of the compounds of the formulae A and B in the mixturesfor TN and STN displays according to the invention results in

[0027] high steepness of the electro-optical characteristic line,

[0028] low temperature dependence of the threshold voltage,

[0029] very fast response times, in particular at low temperatures.

[0030] The compounds of the formulae A and B significantly shorten, inparticular, the response times of TN and STN mixtures whilesimultaneously increasing the steepness and reducing the temperaturedependence of the threshold voltage.

[0031] The mixtures according to the invention are furthermoredistinguished by the following advantages:

[0032] they have low viscosity,

[0033] they have a low threshold voltage and operating voltage, and

[0034] they effect long shelf lives in the display at low temperatures.

[0035] The invention furthermore relates to a liquid-crystal displayhaving

[0036] two outer plates, which, together with a frame, form a cell,

[0037] a nematic liquid-crystal mixture of positive dielectricanisotropy located in the cell,

[0038] electrode layers with alignment layers on the insides of theouter plates,

[0039] a tilt angle between the longitudinal axis of the molecules atthe surface of the outer plates and the outer plates of from 0 degree to30 degrees, and

[0040] a twist angle of the liquid-crystal mixture in the cell fromalignment layer to alignment layer with a value of between 22.5° and6000,

[0041] a nematic liquid-crystal mixture consisting of

[0042] a) 15-75% by weight of a liquid-crystalline component Aconsisting of one or more compounds having a dielectric anisotropy ofgreater than +1.5;

[0043] b) 2-85% by weight of a liquid-crystalline component B consistingof one or more compounds having a dielectric anisotropy of between −1.5and +1.5;

[0044] c) 0-20% by weight of a liquid-crystalline component D consistingof one or more compounds having a dielectric anisotropy of below −1.5,and

[0045] d) if desired, an optically active component C in such an amountthat the ratio between the layer thickness (separation of the outerplates) and the natural pitch of the chiral nematic liquid-crystalmixture is from about 0.2 to 1.3,

[0046] characterised in that component A comprises at least one compoundof the formula A

[0047] and at least one compound of the formula B

[0048] in which R^(a), R^(b), ring A, ring B, a, Z¹, Z², L¹, L², L³, L⁴,L⁵, L⁶, L⁷, L⁸, L⁹ and Y are as defined above.

[0049] The invention also relates to TN and STN displays, in particularmedium- and low-multiplexed STN displays, containing the liquid-crystalmixture according to the invention.

[0050] The use of the compounds of the formulae A and B in IPS mixturesresults in improved response times owing to the reduced rotationalviscosity and in a reduction in the driving voltage for monitor and TVapplications.

[0051] Formula A covers, in particular, compounds of the sub-formulaeA-1 to A-56

[0052] in which R^(a) is as defined above, and (F) represents anoptional F substituent.

[0053] The invention also relates to compounds of the formula A, inparticular of the formulae A-1 to A-24 and A-29 to A-52.

[0054] Of the compounds of the formulae A-1 to A-56, particularpreference is given in the mixtures according to the invention to thecompounds of the formulae A-1, A-2, A-3, A-4, A-7, A-8, A-9, A-10, A-13,A-14, A-15, A-16, A-17, A-18, A-22, A-25, A-26, A-27 and A-28,furthermore A-29, A-30, A-31, A-32, A-35, A-36, A-37, A-38, A41, A-42,A43, A-44, A-45, A-46, A-50, A-53, A-54, A-55 and A-56.

[0055] Particular preference is given to the compounds of the formulaeA-2, A-8, A-15, A-17, A-18 and A-27.

[0056] Particular preference is given to mixtures according to theinvention which comprise at least one compound of the formula A-2 and/orA-8.

[0057] In the formulae A and A-1 to A-12, R^(a) is particularlypreferably straight-chain alkyl or alkoxy, 1 E-alkenyl or 3E-alkenylhaving from 2 to 7 carbon atoms.

[0058] Y is preferably F, Cl, CN, CF₃, C₂F₅, C₃F₇, CF₂H, OCF₃, OCF₂H,OCFHCF₃, OCFHCFH₂, OCFHCF₂H, OCF₂CH₃, OCF₂CFH₂, OCF₂CF₂H, OCF₂CF₂CF₂H,OCF₂CF₂CFH₂, OCFHCF₂CF₃, OCFHCF₂CF₂H, OCFHCFHCF₃, OCH₂CF₂CF₃,OCF₂CF₂CF₃, OCF₂CFHCFH₂, OCF₂CH₂CF₂H, OCFHCF₂CFH₂, OCFHCFHCF₂H,OCFHCH₂CF₃, OCH₂CFHCF₃, OCH₂CF₂CF₂H, OCF₂CFHCH₃, OCF₂CH₂CFH₂,OCFHCF₂CH₃, OCFHCFHCFH₂, OCFHCH₂CF₃, OCH₂CF₂CFH₂, OCH₂CFHCF₂H,OCF₂CH₂CH₃, OCFHCFHCH₃, OCFHCH₂CFH₂, OCH₂CF₂CH₃, OCH₂CFHCFH₂,OCH₂CH₂CF₂H, OCHCH₂CH₃, OCH₂CFHCH₃, OCH₂CH₂CF₂H, OCClFCF₃, OCClFCClF₂,OCClFCFH₂, OCFHCCl₂F, OCClFCF₂H, OCClFCClF₂, OCF₂CClH₂, OCF₂CCl₂H,OCF₂CCl₂F, OCF₂CClFH, OCF₂CClF₂, OCF₂CF₂CClF₂, OCF₂CF₂CCl₂F,OCClFCF₂CF₃, OCClFCF₂CF₂H, OCClFCF₂CClF₂, OCClFCFHCF₃, OCClFCClFCF₃,OCCl₂CF₂CF₃, OCClHCF₂CF₃, OCClFCF₂CF₃, OCClFCClFCF₃, OCF₂CClFCFH₂,OCF₂CF₂CCl₂F, OCF₂CCl₂CF₂H, OCF₂CH₂CClF₂, OCClFCF₂CFH₂, OCFHCF₂CCl₂F,OCClFCFHCF₂H, OCClFCClFCF₂H, OCFHCFHCClF₂, OCClFCH₂CF₃, OCFHCCl₂CF₃,OCCl₂CFHCF₃, OCH₂CClFCF₃, OCCl₂CF₂CF₂H, OCH₂CF₂CClF₂, OCF₂CClFCH₃,OCF₂CFHCCl₂H, OCF₂CCl₂CFH₂, OCF₂CH₂CCl₂F, OCClFCF₂CH₃, OCFHCF₂CCl₂H,OCClFCClFCFH₂, OCFHCFHCCl₂F, OCClFCH₂CF₃, OCFHCCl₂CF₃, OCCl₂CF₂CFH₂,OCH₂CF₂CCl₂F, OCCl₂CFHCF₂H, OCClHCClFCF₂H, OCF₂CClHCClH₂, OCF₂CH₂CCl₂H,OCClFCFHCH₃, OCF₂CClFCCl₂H, OCClFCH₂CFH₂, OCFHCCl₂CFH₂, OCCl₂CF₂CH₃,OCH₂CF₂CClH₂, OCCl₂CFHCFH₂, OCH₂CClFCFCl₂, OCH₂CH₂CF₂H, OCClHCClHCF₂H,OCH₂CCl₂CF₂H, OCClFCH₂CH₃, OCFHCH₂CCl₂H, OCClHCFHCClH₂, OCH₂CFHCCl₂H,OCCl₂CH₂CF₂H, OCH₂CCl₂CF₂H, CH═CF₂, CF═CF₂, OCH═CF₂, OCF═CF₂, CH═CHF,OCH═CHF, CF═CHF, OCF═CHF, in particular F, Cl, CN, CF₃, CF₂H, C₂F₅,C₃F₇, OCF₃, OCF₂H, OCFHCF₃, OCFHCFH₂, OCFHCF₂H, OCF₂CH₃, OCF₂CFH₂,OCF₂CF₂H, OCF₂CF₂CF₂H, OCF₂CF₂CFH₂, OCFHCF₂CF₃, OCFHCF₂CF₂H, OCF₂CF₂CF₃,OCF₂CHFCF₃, OCClFCF₂CF₃.

[0059] Formula B covers, in particular, compounds of the sub-formulaeB-1 to B-6,

[0060] in which R^(b) is as defined above.

[0061] Particular preference is given to compounds in which R^(b) is astraight-chain alkyl radical having 1-7 carbon atoms or an alkenylradical having 2-7 carbon atoms. Particular preference is given tocompounds of the formulae B-1, B-2 and B4.

[0062] The use of compounds of the formulae A and B in theliquid-crystal mixtures according to the invention results inparticularly low rotational viscosity values and in TN and STN displayshaving high steepness and fast response times, in particular at lowtemperatures.

[0063] Besides the compounds of the formula A, component A, or theliquid-crystalline mixture according to the invention, preferablyadditionally comprises one or more 3,4,5-trifluorophenyl compoundsselected from the group consisting of the compounds of the formulae IIato Ilk

[0064] in which

[0065] R² is H, an alkyl radical having from 1 to 12 carbon atoms whichis unsubstituted, monosubstituted by CN or CF₃ or at leastmonosubstituted by halogen, where one or more CH₂ groups in theseradicals may also each, independently of one another, be replaced by—O—, —S—,

[0066]  —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a waythat O atoms are not linked directly to one another.

[0067] Besides the compounds of the formulae A and B, the mediumaccording to the invention may additionally comprise one or morecompounds containing polar end groups, of the formulae II*a to II*s

[0068] in which R² is as defined for R^(a), and L³ and L⁴ are each,independently of one another, H or F. R² in these compounds isparticularly preferably alkyl, alkenyl or alkoxy having up to 7 carbonatoms.

[0069] The medium according to the invention or component A particularlypreferably comprises compounds of the formulae IIa, IIb, IIc, IId, lie,IIf, IIg, IIj, II*b, II*c, II*d, II*f and/or II*i, in particularcompounds of the formulae IIa, IIb, IId, IIi, II*a and II*i.

[0070] Besides one or more compounds of the formula B, the mixtureaccording to the invention preferably comprises one or more cyanocompounds of the formulae IIIa to IIIj:

[0071] in which R³ is as defined for R^(a), and L¹, L² and L⁵ are each,independently of one another, H or F. R³ in these compounds isparticularly preferably alkyl, alkenyl or alkoxy having up to 7 carbonatoms.

[0072] Particular preference is given to mixtures which comprise one ormore compounds of the formulae IIIb, IIIc, IIIf and IIIj, in particularthose in which L¹ and/or L² is F.

[0073] Preference is furthermore given to mixtures which comprise one ormore compounds of the formula IIIf and/or IIIg in which L² is H and L¹is H or F, in particular F.

[0074] The individual compounds of the formulae A, B and of the formulaeIIa-IIk, II*a-II*s and IIIa to IIIj and their sub-formulae or also othercompounds which can be used in the mixtures or TN and STN displaysaccording to the invention are either known or can be preparedanalogously to the known compounds.

[0075] The compounds of the formula A have low viscosities, inparticular low rotational viscosities, and low values for the ratio ofthe elastic constants K₃₃/K₁₁, and therefore result in short responsetimes in the displays according to the invention, while the presence ofcompounds of the formula B of high dielectric anisotropy, in particularin increased concentrations, causes a reduction in the viscosity.

[0076] Preferred liquid-crystal mixtures comprise component A in aproportion of from 15% to 75%, particularly preferably from 20% to 65%.The compounds of component A preferably have a dielectric anisotropy ofΔε≧+3, in particular of Δε≧+8, particularly preferably of Δε≧+12.

[0077] Further preferred mixtures comprise

[0078] one or more, in particular from two to four, compounds of theformula A,

[0079] in each case one, two or three compounds of the formula A,

[0080] one or more, in particular one or two, compounds of the formulaB,

[0081] one or more, in particular from two to five, compounds of theformulae IIIa, IIIb, IIIc and/or IIIf.

[0082] Preferred liquid-crystal mixtures comprise component B in aproportion of from 2 to 85%, particularly preferably 5 to 80%,especially 5 to 75%. The compounds from group B are distinguished, inparticular, by their low values for the rotational viscosity γ₁.

[0083] Component B preferably comprises one or more compounds of theformula IV

[0084] in which

[0085] m is 0 or 1,

[0086] R⁴ is an alkenyl group having from 2 to 7 carbon atoms,

[0087] R⁵ is as defined for R^(a) or, if m=1, is alternatively F, Cl,CF₃ or OCF₃,

[0088] L¹ and L² are each, independently of one another, H or F.

[0089] Particularly preferred compounds of the formula IV are those inwhich R⁴ is alkenyl having from 2 to 7 carbon atoms, in particular thoseof the following formulae:

[0090] in which R^(4a) and R^(5a) are each, independently of oneanother, H, CH₃, C₂H₅ or n-C₃H₇, and alkyl is an alkyl group having from1 to 7 carbon atoms.

[0091] Particular preference is given to TN and STN displays accordingto the invention in which the liquid-crystal mixture comprises at leastone compound of the formula IV-1 and/or IV-3 in which R^(4a) and R^(5a)each have the same meaning, and displays in which the liquid-crystalmixture comprises at least one compound of the formula IV-5.

[0092] In a further preferred embodiment, the mixtures according to theinvention comprise one or more compounds of the formula IV-6.

[0093] Component B preferably furthermore comprises compounds selectedfrom the group consisting of the bicyclic compounds of the formulae V-1to V-9

[0094] and/or one or more compounds selected from the group consistingof the tricyclic compounds of the formulae V-10 to V-27

[0095] and/or one or more compounds selected from the group consistingof the tetracyclic compounds of the formulae V-28 to V-34

[0096] in which R⁶ and R⁷ are as defined for Ra in the formula A, and Lis H or F.

[0097] Particular preference is given to compounds of the formulae V-25to V-34 in which R⁶ is alkyl and R⁷ is alkyl or alkoxy, in particularalkoxy, each having from 1 to 7 carbon atoms. Preference is furthermoregiven to compounds of the formulae V-25, V-28 and V-34 in which L is F.

[0098] R⁶ and R⁷ in the compounds of the formulae V-1 to V-34 areparticularly preferably straight-chain alkyl or alkoxy having from 1 to12 carbon atoms.

[0099] Particular preference is given to mixtures according to theinvention which comprise one or more compounds of the formula B-3aand/or B-5a

[0100] The mixtures preferably comprise 2-25% by weight, in particular2-15% by weight, of compounds of the formula B-5a.

[0101] If desired, the liquid-crystalline mixtures may comprise anoptically active component C in such an amount that the ratio betweenthe layer thickness (separation of the outer plates) and the naturalpitch of the chiral nematic liquid-crystal mixture is greater than 0.2.A multiplicity of chiral dopants, some of which are commerciallyavailable, is available to the person skilled in the art for thecomponent, such as, for example, cholesteryl nonanoate, S-811, S-1011,S-2011, S-4011, and S-5011 from Merck KGaA, Darmstadt, and CB15 (BDH,Poole, UK). The choice of dopants is not crucial per se.

[0102] The proportion of the compounds of component C is preferably from0 to 10%, in particular from 0 to 5%, particularly preferably from 0 to3%.

[0103] The mixtures according to the invention may also, if desired,comprise up to 20% of one or more compounds having a dielectricanisotropy of less than −2 (component D).

[0104] If the mixtures comprise compounds of component D, these arepreferably one or more compounds containing the structural unit2,3-difluoro-1,4-phenylene, for example compounds as described in DE-A38 07 801, 38 07 861, 38 07 863, 38 07 864 or 38 07 908. Particularpreference is given to tolans containing this structural unit, asdescribed in International Patent Application PCT/DE 88/00133.

[0105] Further known compounds of component D are, for example,derivatives of the 2,3-dicyanohydroquinones or cyclohexane derivativescontaining the structural unit

[0106] as described in DE-A 32 31 707 and DE-A 34 07 013.

[0107] The liquid-crystal displays according to the invention preferablycontain no compounds of component D.

[0108] The term “alkenyl” in the definition of R^(a), R^(b), R², R³, R⁴,R⁵, R⁶ and R⁷ covers straight-chain and branched alkenyl groups, inparticular the straight-chain groups. Particularly preferred alkenylgroups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl, C₅-C₇-4-alkenyl,C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particular C₂-C₇-1E-alkenyl,C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl.

[0109] Examples of preferred alkenyl groups are vinyl, 1E-propenyl,1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl,3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

[0110] Further preferred embodiments relate to liquid-crystal mixturesaccording to the invention which

[0111] additionally comprise one or more, particularly preferably one,two or three, heterocyclic compounds of the formula Va and/or Vb

[0112] in which

[0113] R⁶ and R⁷ are as defined above,

[0114] and

[0115] Y is F or Cl,

[0116] where the proportion of the compounds from the group consistingof Va and Vb is preferably from 2 to 35%, in particular from 5 to 20%,

[0117] additionally comprise one or more, particularly preferably one,two or three, tolan compounds of the formulae T2a, T2b and/or T2c

[0118] in which R⁶ and R⁷ are as defined above.

[0119] The proportion of the compounds from the group consisting of T2a,T2b and/or T2c is preferably from 2 to 20%, in particular from 4 to 12%.The mixture according to the invention preferably comprises two or threecompounds of the formula T2a and/or T2b.

[0120] In particularly preferred embodiments, the mixtures comprise

[0121] at least one compound of the formula A-2

[0122] and

[0123] at least one compound of the formula B-1

[0124] at least one compound of the formula Iij;

[0125] at least one tolan compound of the formula T2c;

[0126] at least two, in particular three, compounds of the formula A;

[0127] at least two, in particular three, compounds of the formula B;

[0128] at least one compound of the formula T2a and at least onecompound of the formula T2b;

[0129] at least one compound of the formula VI

[0130] in which L¹ is H or F;

[0131] at least one compound of the formula VII

[0132] in which alkyl and alkyl* are each, independently of one another,an alkyl group having from 1 to 7 carbon atoms.

[0133] at least 2 to 30% by weight, preferably 2-25% by weight, inparticular from 3 to 15% by weight, of compounds of the formula VII

[0134] at least 2.5% by weight of the compounds of the formula IV-5;

[0135] 5-30% by weight, preferably 10-25% by weight, of the compounds ofthe formula A;

[0136] 3-30% by weight, preferably 3-20% by weight, of the compounds ofthe formula B;

[0137] at least three homologues of the compounds of the formula A,where Ra is then preferably C₂H₅, n-C₃H₇ and n-C₅H₁₁.

[0138] Further particularly preferred embodiments relate toliquid-crystal mixtures which comprise

[0139] a total of from three to six compounds of the formulae A and B,where the proportion of these compounds with respect to the mixture as awhole is from 25 to 65%, in particular from 30 to 55%,

[0140] more than 20% of compounds of positive dielectric anisotropy, inparticular having Δε≧+12.

[0141] The mixtures according to the invention are distinguished, inparticular on use in TN and STN displays of high layer thicknesses, byvery low total response times (t_(tot)=t_(on)+t_(off)).

[0142] The liquid-crystal mixtures used in the TN and STN cellsaccording to the invention are dielectrically positive, with Δε≧1.Particular preference is given to liquid-crystal mixtures with Δε≧3, inparticular with Δε≧5.

[0143] The liquid-crystal mixtures according to the invention havefavourable values for the threshold voltage V_(10/0/20) and for therotational viscosity γ₁. If the value for the optical path differenced·Δn is pre-specified, the value for the layer thickness d is determinedby the optical anisotropy Δn. In particular at relatively high valuesfor d·Δn, the use of liquid-crystal mixtures according to the inventionhaving a relatively high value for the optical anisotropy is generallypreferred, since the value for d can then be selected to be relativelysmall, which results in more favourable values for the response times.However, liquid-crystal displays according to the invention whichcontain liquid-crystal mixtures according to the invention with smallervalues for Δn are also characterised by advantageous values for theresponse times.

[0144] The liquid-crystal mixtures according to the invention arefurthermore characterised by advantageous values for the steepness ofthe electro-optical characteristic line, and can be operated with highmultiplex rates, in particular at temperatures above 20° C. In addition,the liquid-crystal mixtures according to the invention have highstability and favourable values for the electrical resistance and thefrequency dependence of the threshold voltage. The liquid-crystaldisplays according to the invention have a large working-temperaturerange and good angle dependence of the contrast.

[0145] The construction of the liquid-crystal display elements accordingto the invention from polarisers, electrode base plates and electrodeshaving a surface treatment such that the preferential alignment(director) of the liquid-crystal molecules in each case adjacent theretois usually twisted by a value of from 160° to 720° from one electrode tothe other corresponds to the usual structure for display elements ofthis type. The term “usual structure” here is broadly drawn and alsocovers all derivatives and modifications of the TN and STN cell, inparticular also matrix display elements and display elements containingadditional magnets.

[0146] The surface tilt angle at the two outer plates may be identicalor different. Identical tilt angles are preferred. Preferred TN displayshave pre-tilt angles between the longitudinal axis of the molecules atthe surface of the outer plates and the outer plates of from 0° to 70,preferably from 0.010 to 5°, in particular from 0.1 to 20. In the STNdisplays, the pre-tilt angle is from 10 to 30°, preferably from 1° to12° and in particular from 3° to 10°.

[0147] The twist angle of the TN mixture in the cell has a value ofbetween 22.5° and 170°, preferably between 45° and 130° and inparticular between 80° and 115°. The twist angle of the STN mixture inthe cell from alignment layer to alignment layer has a value of between100° and 600°, preferably between 170° and 300° and in particularbetween 180° and 270°.

[0148] The liquid-crystal mixtures which can be used in accordance withthe invention are prepared in a manner which is conventional per se. Ingeneral, the desired amount of the components used in lesser amount aredissolved in the components making up the principal constituent,advantageously at elevated temperature. It is also possible to mixsolutions of the components in an organic solvent, for example inacetone, chloroform or methanol, and to remove the solvent again, forexample by distillation, after thorough mixing.

[0149] The dielectrics may also comprise further additives which areknown to the person skilled in the art and are described in theliterature. For example, 0-15% of pleochroic dyes, stabilisers, such as,for example, Tinuvin® from Ciba, antioxidants, UV absorbers, etc., maybe added.

[0150] In the present application and in the examples below, thestructures of the liquid-crystal compounds are indicated by means ofacronyms, the transformation into chemical formulae taking place inaccordance with Tables A and B. All radicals C_(n)H_(2n+1) andC_(m)H_(2m+1) are straight-chain alkyl radicals having n and m carbonatoms respectively. The alkenyl radicals have the trans-configuration.The coding in Table B is self-evident. In Table A, only the acronym forthe parent structure is indicated. In individual cases, the acronym forthe parent structure is followed, separated by a dash, by the codeindicated in the table below for the substituents R^(1*), R^(2*),L^(1*), L^(2*) and L^(3*.)

[0151] In the foregoing and in the following examples, all temperaturesare set forth uncorrected in degrees Celsius; and, unless otherwiseindicated, all parts and percentages are by weight.

[0152] The entire disclosure of all applications, patents andpublications, cited above and below, and of corresponding GermanApplication No. 10260517.3, filed Dec. 21, 2002 is hereby incorporatedby reference. Code for R¹*, R²*, L¹*, L²*, L³* R¹* R²* L¹* L²* L³* nmC_(n)H_(2n+1) C_(m)H_(2m+1) H H H nOm OC_(n)H_(2n+1) C_(m)H_(2m+1) H H HnO.m C_(n)H_(2n+1) OC_(m)H_(2m+1) H H H n C_(n)H_(2n+1) CN H H H nN.FC_(n)H_(2n+1) CN H H F nN.F.F C_(n)H_(2n+1) CN H F F nF C_(n)H_(2n+1) FH H H nOF OC_(n)H_(2n+1) F H H H nF.F C_(n)H_(2n+1) F H H F nmFC_(n)H_(2n+1) C_(m)H_(2m+1) F H H nOCF₃ C_(n)H_(2n+1) OCF₃ H H H n-VmC_(n)H_(2n+1) —CH═CH—C_(m)H_(2m+1) H H H nV-Vm C_(n)H_(2n+1)—CH═CH——CH═CH—C_(m)H_(2m+1) H H H n.F.F.F C_(n)H_(2n+1) F H F F

[0153] The displays, in particular TN-, STN- and IPS displays,preferably contain liquid-crystalline mixtures composed of one or morecompounds from Tables A and B. TABLE A (L^(1*), L^(2*), L^(3*) = H or F)

BCH

CBC

CH

CCP

CPTP

PTP

ECCP

EPCH

CP

ME

HP

PCH

CCPC

CCH

PDX-n

[0154] TABLE B

CBC-nmF

DU-n-N

CP-nmF

CCZU-n-F

CC-nV-Vm

CC-n-V

CCP-Vn-m

CCG-V-F

CCP-nV-m

PYP-nF

PYP-nm

PTP-nOm

CDU-n-F

PCH-nCl

PPTUI-n-m

PGU-n-F

CGU-n-F

CVCP-V-n

CVCP-V-On

PZU-V2-N

CVCP-1V-On

DCU-n-F

CCOC-n-m

ACU-n-F

CAU-n-F

APG-n-F

APG-n-OT

ACQU-n-F

APU-n-F

APU-n-OT

AU-n-N

CC-n-Vm

AG-n-N

[0155] TABLE C Table C shows dopants which can be added to the mixturesaccording to the invention. The mixtures preferably comprise from 0.01to 10% by weight of dopant.

C 15

CB 15

CM 21

R/S-811

CM 44

CM 45

CM 47

CN

R/S-1011

R/S-2011

R/S-3011

R/S-4011

R/S-5011

[0156] TABLE D Stabilisers which can be added, for example, to themixtures according to the invention are shown below:

[0157] The following examples are intended to illustrate the inventionwithout re-resenting a limitation. The following abbreviations are used:

[0158] cl.p. clearing point (nematic-isotropic phase transitiontemperature),

[0159] S—N smectic-nematic phase transition temperature,

[0160] ν₂₀ flow viscosity (mm²/s, unless stated otherwise, at 20° C.),

[0161] Δn optical anisotropy (589 nm, 20° C.)

[0162] Δε dielectric anisotropy (1 kHz, 20° C.) γ₁ rotational viscosity(mPa·s at 20° C.)

[0163] steep characteristic line steepness=(V₉₀/V₁₀−1). 100 [%]

[0164] V₁₀ threshold voltage=characteristic voltage at a relativecontrast of 10%,

[0165] V₉₀ characteristic voltage at a relative contrast of 90%,

[0166] t_(ave)$\frac{t_{on} + t_{off}}{2}\quad ( {{average}\quad {response}\quad {time}} )$

[0167]  (average response time)

[0168] t_(on) time from switching on until 90% of the maximum contrastis reached,

[0169] t_(off) time from switching off until 10% of the maximum contrastis reached,

[0170] mux multiplex rate

[0171] t_(store) low temperature storage stability in hours (−20° C.,−30° C., −40° C.)

[0172] Above and below, all temperatures are given in ° C. Thepercentages are percent by weight. All values relate to 20° C., unlessstated otherwise. The displays are addressed, unless stated otherwise,without multiplexing.

EXAMPLE 1

[0173] ME2N.F 6.00% Clearing point [° C.]: 95.0 ME3N.F 7.00% Δn [589 nm,20° C.]: 0.1419 ME4N.F 11.00% Δε [1 kHz, 20° C.]: 28.3 ME5N.F 10.00% γ₁[20° C.; mPa · s]: 334 CGU-2-F 8.00% Twist [°]: 240 CGU-3-F 8.00%CCZU-2-F 4.00% CCZU-3-F 10.50% CCZU-5-F 4.00% CCPC-33 2.00% CCPC-342.00% CBC-33F 5.00% CBC-53F 5.00% CBC-55F 5.00% CPTP-301 3.50% APU-3-F9.00%

EXAMPLE 2

[0174] PCH-3N.F.F 12.00% Clearing point [° C.]: 106.0 ME2N.F 2.00% Δn[589 nm, 20° C.]: 0.1388 ME3N.F 3.00% Δε [1 kHz, 20° C.]: 11.9 CGU-2-F5.00% γ₁ [20° C.; mPa · s]: 195 CGU-3-F 5.00% Twist [°]: 240 APU-3-F7.00% CCP-2F.F.F 4.00% CAU-3-F 4.00% CC-5-V 2.00% CCP-V-1 9.00% CCP-V2-15.00% CCG-V-F 8.00% CVCP-1V-O1 4.00% PTP-102 4.00% PTP-201 4.00% PTP-3014.00% PTP-302 3.00% CCPC-33 5.00% CCPC-34 5.00% CCPC-35 5.00%

EXAMPLE 3

[0175] PCH-3N.F.F 8.00% Clearing point [° C.]: 79.0 ME2N.F 10.00% Δn[589 nm, 20° C.]: 0.1314 ME3N.F 10.00% Δε [1 kHz, 20° C.]: 45.0 ME4N.F11.00% γ₁ [20° C.; mPa · s]: 397 ME5N.F 10.00% Twist [°]: 240 HP-3N.F5.00% HP-4N.F 5.00% HP-5N.F 4.00% CCP-2F.F.F 5.00% CCP-3F.F.F 5.00%CCP-5F.F.F 5.00% CCPC-33 4.00% CCPC-34 4.00% CCPC-35 4.00% ACU-3-F10.00%

Example 4

[0176] PCH-3N.F.F 8.00% Clearing point [° C.]: 79.0 ME2N.F 10.00% Δn[589 nm, 20° C.]: 0.1312 ME3N.F 10.00% Δε [1 kHz, 20° C.]: 45.0 ME4N.F11.00% γ₁ [20° C.; mPa · s]: 385 ME5N.F 10.00% HP-3N.F 5.00% HP-4N.F5.00% HP-5N.F 4.00% CCP-2F.F.F 5.00% CCP-3F.F.F 6.00% CCP-5F.F.F 5.00%CCPC-33 4.00% CCPC-34 3.00% CCPC-35 4.00% ACQU-3-F 10.00%

EXAMPLE 5

[0177] PCH-3N.F.F 8.00% Clearing point [° C.]: 79.5 ME2N.F 10.00% Δn[589 nm, 20° C.]: 0.1326 ME3N.F 10.00% Δε [1 kHz, 20° C.]: 44.7 ME4N.F11.00% γ₁ [20° C.; mPa · s]: 399 ME5N.F 10.00% HP-3N.F 5.00% HP-4N.F5.00% HP-5N.F 4.00% CCP-2F.F.F 5.00% CCP-3F.F.F 6.00% CCP-5F.F.F 5.00%CCPC-33 3.00% CCPC-34 4.00% CCPC-35 4.00% CAU-3-F 10.00%

EXAMPLE 6

[0178] PCH-3N.F.F 8.00% Clearing point [° C.]: 80.0 ME2N.F 10.00% Δn[589 nm, 20° C.]: 0.1389 ME3N.F 10.00% Δε [1 kHz, 20° C.]: 45.2 ME4N.F11.00% γ₁ [20° C.; mPa · s]: 403 ME5N.F 10.00% HP-3N.F 5.00% HP-4N.F5.00% HP-5N-F 4.00% CCP-2F.F.F 5.00% CCP-3F.F.F 5.00% CCP-5F.F.F 5.00%CCPC-33 4.00% CCPC-34 4.00% CCPC-35 4.00% APU-3-F 10.00%

EXAMPLE 7

[0179] PCH-3N.F.F 8.00% Clearing point [° C.]: 79.0 ME2N.F 10.00% Δn[589 nm, 20° C.]: 0.1382 ME3N.F 10.00% Δε [1 kHz, 20° C.]: 46.0 ME4N.F11.00% γ₁ [20° C.; mPa · s]: 406 ME5N.F 10.00% HP-3N.F 5.00% HP-4N.F5.00% HP-5N.F 4.00% CCP-2F.F.F 5.00% CCP-3F.F.F 6.00% CCP-5F.F.F 5.00%CCPC-33 3.00% CCPC-34 4.00% CCPC-35 4.00% APU-3-OT 10.00%

EXAMPLE 8

[0180] PCH-3N.F.F 8.00% Clearing point [° C.]: 79.0 ME2N.F 10.00% Δn[589 nm, 20° C.]: 0.1377 ME3N.F 10.00% Δε [1 kHz, 20° C.]: 45.1 ME4N.F11.00% γ₁ [20° C.; mPa · s]: 391 ME5N.F 10.00% HP-3N.F 5.00% HP-4N.F5.00% HP-5N.F 4.00% CCP-2F.F.F 5.00% CCP-3F.F.F 7.50% CCP-5F.F.F 5.00%CCPC-33 3.00% CCPC-34 3.50% CCPC-35 3.00% APG-3-OT 10.00%

EXAMPLE 9

[0181] ME2N.F 3.00% Clearing point [° C.]: 78.0 PZU-V2-N 5.00% Δn [589nm, 20° C.]: 0.1078 AU-3-N 7.00% Δε [1 kHz, 20° C.]: 10.9 PGU-2-F 9.00%γ₁ [20° C.; mPa · s]: 85 PGU-3-F 6.00% CCP-30CF₃ 8.00% CCP-40CF₃ 2.00%CC-3-V1 12.00% CC-5-V 14.00% PCH-302 8.00% CCP-V-1 15.00% CCP-V2-111.00%

EXAMPLE 10

[0182] ME2N.F 3.00% Clearing point [° C.]: 78.0 PZU-V2-N 5.00% Δn [589nm, 20° C.]: 0.1075 AG-3-N 9.00% Δε [1 kHz, 20° C.]: 10.8 PGU-2-F 9.50%γ₁ [20° C.; mPa · s]: 87 PGU-3-F 5.00% CCP-30CF₃ 8.00% CCP-40CF₃ 4.00%CC-3-V1 12.00% CC-5-V 15.00% PCH-302 5.50% CCP-V-1 15.00% CCP-V2-1 9.00%

EXAMPLE 11

[0183] CCP-20CF₃ 7.00% Clearing point [° C.]: 75.0 CCP-30CF₃ 7.00% Δn[589 nm, 20° C.]: 0.0750 PDX-3 6.00% Δε [1 kHz, 20° C.]: 9.2 PDX-4 4.00%γ₁ [20° C.; mPa · s]: 95 ME2N.F 2.50% CCZU-2-F 7.00% CCZU-3-F 15.00% CH-33 1.50% CH-35 2.00% CH-43 2.00% CC-5-V 18.00%  CC-3-V1 11.00% PCH-302 2.50% ACU-2-F 7.00% ACU-3-F 7.50%

EXAMPLE 12

[0184] ACU-2-F 10.00%  Clearing point [° C.]: 73.5 ACQU-3-F 11.00%  Δn[589 nm, 20° C.]: 0.0782 CCP-20CF₃ 7.00% Δε [1 kHz, 20° C.]: 18.4CCP-30CF₃ 8.00% γ₁ [20° C.; mPa · s]: 137 CP-30CF₃ 7.00% CP-50CF₃ 7.00%DU-3-N 12.50%  ME2N.F 3.00% ME3N.F 2.00% CCZU-2-F 4.00% CCZU-3-F 15.00% CCZU-4-F 6.00% CC-3-V1 2.50% CCH-35 5.00%

EXAMPLE 13

[0185] ACU-2-F 9.00% Clearing point [° C.]: 70.0 ACU-3-F 9.00% Δn [589nm, 20° C.]: 0.0754 ACQU-3-F 9.00% Δε [1 kHz, 20° C.]: 10.1 CCP-20CF₃3.00% γ₁ [20° C.; mPa · s]: 83 CCP-30CF₃ 8.00% CP-30CF₃ 7.00% CP-50CF₃6.00% PCH-3N.F.F 10.00%  ME2N.F 4.00% ME3N.F 2.00% CCZU-2-F 4.00%CCZU-3-F 15.00%  CCZU-5-F 3.00% CC-5-V 11.00% 

[0186] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

[0187] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A liquid-crystalline medium comprising one or more compounds offormula A

and at least one compound of formula B

in which R^(a) and R^(b) are each, independently of one another, H or analkyl radical having from 1 to 12 carbon atoms which is unsubstituted,monosubstituted by CN or CF₃ or at least monosubstituted by halogen, andin which one or more CH₂ groups are each, independently of one another,optionally replaced by —O—, —S—,

 —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that Oatoms are not linked directly to one another,

 are each, independently of one another,

and at least one ring is

Z¹ and Z² are each, independently of one another, —CH₂CH₂—, —(CH₂)₄—,—CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, —C═C—, —CF═CF—, —CH═CH—, —COO—, —CH₂—,—(CH₂)₃— or a single bond, a is 0 or 1, L¹ to L⁹ are each, independentlyof one another, H or F, and Y is F, C₁, SF₅, NCS, SCN, CN, OCN, or amonohalogenated or polyhalogenated alkyl, alkoxy, alkenyl or alkenyloxyradical, in each case having up to 5 carbon atoms.
 2. Aliquid-crystalline medium according to claim 1, wherein said mediumcontains at least one compound of formulae A-1 to A-56

in which (F) is H or F.
 3. A liquid-crystalline medium according toclaim 1 or 2, wherein said medium contains at least one compound offormulae B-1 to B-6


4. A liquid-crystal medium according to one of claims 1 to 3, furthercomprising at least one compound of formulae IIa to Ilk

in which R² is H, an alkyl radical having from 1 to 12 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or at leastmonosubstituted by halogen, and in which one or more CH₂ groups are,independently of one another, optionally replaced by —O—, —S—,

 —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that Oatoms are not linked directly to one another.
 5. A liquid-crystal mediumaccording to one of claims 1 to 4, further comprising one or more cyanocompounds of formulae IIIa to IIIj

in which R³ is an alkyl radical having from 1 to 12 carbon atoms whichis unsubstituted, monosubstituted by CN or CF₃ or at leastmonosubstituted by halogen, and in which one or more CH₂ groups are,independently of one another, optionally replaced by —O—, —S—,

 —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that Oatoms are not linked directly to one another, and L¹, L² and L⁵ areeach, independently of one another, H or F.
 6. A liquid-crystal mediumaccording to one of claims 1 to 5, further com prising one or morecompounds of formula IV

in which m is 0 or 1, R⁴ is an alkenyl group having from 2 to 7 carbonatoms, R⁵ is H or an alkyl radical having from 1 to 12 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or at leastmonosubstituted by halogen, and in which one or more CH₂ groups areeach, independently of one another, optionally replaced by —O—, —S—,

—CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that Oatoms are not linked directly to one another, or, if m is 1, R⁵ can alsobe F, Cl, CF₃ or OCF₃, and L¹ and L² are each, independently of oneanother, H or F.
 7. A liquid-crystalline medium according to one ofclaims 1 to 6, further comprises one or more compounds of formula VII

in which P1 alkyl and alkyl* are each, independently of one another, analkyl group having from 1 to 7 carbon atoms.
 8. A liquid-crystallinemedium according to one of claims 1 to 7, further comprises one or moretolan compounds selected from formulae T2a, T2b and T2c

in which R⁶ and R⁷ are an alkyl radical having from 1 to 12 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or at leastmonosubstituted by halogen, and in which one or more CH₂ groups are,independently of one another, optionally replaced by —O—, —S—,

 —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that Oatoms are not linked directly to one another.
 9. A liquid-crystallinemedium according to one of claims 1 to 8, wherein said medium contains5-30% by weight of compounds of the formula A.
 10. A liquid-crystallinemedium according to one of claims 1 to 9, wherein said medium contains3-30% by weight of compounds of the formula B.
 11. A liquid-crystallinemedium according to one of claims 1 to 10, wherein said medium containsmore than 20% of compounds having a dielectric anisotropy of Δε≧+12. 12.A liquid-crystalline medium according to 10, wherein said mediumcontains more than 20% of compounds having a dielectric anisotropy ofΔε≧+12.
 13. A method of generating an electrooptical effect comprisingactivating an electrooptical device containing a liquid-crystallinemedium according to claim
 1. 14. A method according to claim 12, whereinsaid device is a TN, STN or IPS device.
 15. An electro-opticalliquid-crystal display containing a liquid-crystalline medium accordingto claim
 1. 16. A liquid-crystalline compound of formula A

in which R^(a) is H or an alkyl radical having from 1 to 12 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or at leastmonosubstituted by halogen, and in which one or more CH₂ groups areeach, independently of one another, optionally replaced by —O—, —S—,

 —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that Oatoms are not linked directly to one another,

 are each, independently of one another,

 and at least one ring is

a is 0 or 1, L¹ and L² are each, independently of one another, H or F, Yis F, Cl, SF₅, NCS, OCN, SCN, or a monohalogenated or polyhalogenatedalkyl, alkoxy, alkenyl or alkenyloxy radical, in each case having from 1to 5 carbon atoms, and Z¹ and Z² are each, independently of one another,—CH₂CH₂—, —(CH₂)₄—, —CH₂O—, —OCH₂—, —OCF₂—, —C≡C—, —CF═CF—, —CH═CH—,—COO—, —CH₂—, —(CH₂)₃— or a single bond.
 17. A liquid-crystallinecompound according to claim 16, wherein said compound is selected fromformulae A-1 to A-24 and formulae A-29 to A-52,

wherein (F) is H or F.
 18. A TN or STN liquid-crystal display comprisingtwo outer plates, which, together with a frame, form a cell, a nematicliquid-crystal mixture of positive dielectric anisotropy located in saidcell, electrode layers with alignment layers on the insides of saidouter plates, a tilt angle between the longitudinal axis of themolecules at the surface of the outer plates and the outer plates of0-30 degrees, and a twist angle of said liquid-crystal mixture in thecell from alignment layer to alignment layer with a value of 22.5°-600°,wherein said a nematic liquid-crystal mixture comprises a) 15-75% byweight of a liquid-crystalline component A consisting of one or morecompounds having a dielectric anisotropy of greater than +1.5; b) 2-85%by weight of a liquid-crystalline component B consisting of one or morecompounds having a dielectric anisotropy of between −1.5 and +1.5; c)0-20% by weight of a liquid-crystalline component D consisting of one ormore compounds having a dielectric anisotropy of below −1.5, and d)optionally, an optically active component C in an amount whereby theratio between the layer thickness and the natural pitch of the chiralnematic liquid-crystal mixture is 0.2 to 1.3; and wherein component Acomprises at least one compound of formula A

and at least one compound of formula B

in which R^(a) and R^(b) are each, independently of one another, H or analkyl radical having from 1 to 12 carbon atoms which is unsubstituted,monosubstituted by CN or CF₃ or at least monosubstituted by halogen, andin which one or more CH₂ groups are, independently of one another,optionally replaced by —O—, —S—,

 —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that Oatoms are not linked directly to one another,

 are each, independently of one another,

 and at least one ring is

Z¹ and Z² are each, independently of one another, —CH₂CH₂—, —(CH₂)₄—,—CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, —C≡C—, —CF═CF—, —CH═CH—, —COO—, —CH₂—,—(CH₂)₃— or a single bond, a is 0 or 1, L¹ to L⁹ are each, independentlyof one another, H or F, and Y is F, Cl, SF₅, NCS, OCN, SCN, or amonohalogenated or polyhalogenated alkyl, alkoxy, alkenyl or alkenyloxyradical, in each case having 1 to 5 carbon atoms.